Crystal force and pressure transducers

Abstract

A force or fluid pressure transducer comprises a plate-like crystal. The force, to be measured, is applied by two seatings disposed on opposite edges of the crystal. The transducer can measure pressure when a diaphragm is added to provide the force. Two portions of the crystal are maintained in continuous oscillation by feed-back circuits. These portions change their frequencies by different amounts when the force is changed. The difference between the two frequencies is a very accurate measure of the force and forms the output of the transducer. Structures to secure the crystal have, in the past, proved difficult to manufacture. The present invention describes a securing structure for the crystal which substantially improves and facilitates securing and housing the crystal to provide high accuracy under all normal working conditions of the transducer.

I claim: 
     
       1. A force and pressure transducer comprising: a plate-shaped piezo-electric crystal having a peripheral edge,   a first pair of electrodes disposed on a corresponding region on opposite sides of a first portion of the crystal,   a second pair of electrodes disposed on corresponding regions on opposite sides of a second portion of the crystal,   means for energising the electrode pairs to maintain oscillation of the first and second crystal portions at difference frequencies,   a U-shaped frame containing inside the bottom of its U-shape a first seating member engaging the peripheral edge of the crystal,   a substantially flat leaf spring bridging the gap at the open end of the U-shaped frame and having on it a second seating member engaging the peripheral edge of the crystal and arranged to apply a force along a line extending through said first oscillating crystal portion to said first seating member.   
     
     
       2. A force and pressure transducer according to claim 1, including means for computing the difference frequency between the two oscillating portions and making this difference frequency available as the transducer output. 
     
     
       3. A force and pressure transducer according to claim 1 the pair of seating members being arranged in positions along the crystal periphery to cause the force sensitivity of the transducer output to be independent of temperature changes. 
     
     
       4. A force and pressure transducer according to claim 1, in which said first and second crystal portions are arranged to oscillate in an overtone mode. 
     
     
       5. A force and pressure transducer according to claim 1 in which said first crystal portion or said second crystal portion is arranged to oscillate in an overtone mode. 
     
     
       6. A force and pressure transducer according to claim 1 in which said first and second crystal portions are arranged to oscillate in different overtone modes. 
     
     
       7. A force and pressure transducer according to claim 1 in which the crystal is surrounded by a hermetically sealed enclosure. 
     
     
       8. A force and pressure transducer according to claim 1 in which said enclosure is filled with an inert gas. 
     
     
       9. A force and pressure transducer according to claim 1 in which said enclosure provides a vacuum in which the crystal resonates. 
     
     
       10. A force and pressure transducer according to claim 1 which is arranged to measure a single force. 
     
     
       11. A force and pressure according transducer to claim 1 which is arranged to measure the difference between two forces. 
     
     
       12. A force and pressure transducer according to claim 1 which is arranged to measure a single pressure. 
     
     
       13. A force and pressure transducer according to claim 1 which is arranged to measure the difference between two pressures. 
     
     
       14. A force and pressure transducer according to claim 1 in incorporating over-load features. 
     
     
       15. A force and pressure transducer according to claim 1 in which the force originates from a mass secured in such a manner as to apply the force due to acceleration of said mass to form an accelerometer. 
     
     
       16. A force and pressure transducer according to claim 1 in which undesired variation in frequency of the output signal with temperature is compensated by mass loading of the crystal plate. 
     
     
       17. A force and pressure transducer according to claim 1 in which an adjusting screw is provided to influence the magnitude of the force exerted by the spring upon the crystal. 
     
     
       18. A force and pressure transducer to claim 1 in which the frequency variation with temperature of said oscillating portions is employed to compensate the transducer output for the effect of temperature variation of the transducer. 
     
     
       19. A force and pressure transducer according to claim 1 in which the electrical connections to the resonators are additionally employed to secure the crystal in its position under shock and vibration conditions. 
     
     
       20. A force and pressure transducer according to claim 1 in which said U-shaped frame is supported within said hermetically sealed enclosure by damping material in such a manner as to provide shock resistance of the completed transducer.